Biaxially oriented polypropylene film laminations are commonly used in the snack food packaging industry. In particular, potato chip packaging is a very large volume application. Typically, for this application a metallized, sealable, biaxially oriented polypropylene film is used as the inside layer of the lamination. This film is then laminated to a printed biaxially oriented sealable slip film using a polyethylene extrudate to bond the two films together. The lamination is then shipped to the potato chip manufacturer for vertical form-fill-and-seal (VFFS) packaging.
It is required that the finished package product have the following characteristics: 1) Strong heat seals; 2) Excellent barrier; 3) Excellent lamination bond strength; 4) Excellent print quality; 5) Excellent gloss or surface aesthetics.
Recently, it has been found that at elevated temperature storage or transporting conditions such as might be encountered during hot summer months, these packages can become distorted. This distortion appears to be a result of food oil absorption by the polyolefin film from the potato chips. The surface distortion effectively destroys the surface gloss of the package and gives the appearance that the package has been handled and abused. This distortion is undesirable to snack food manufacturers as they would like to present the product as being fresh or new to the shelf. In addition, it is also important that the surface give a good representation of the food product inside through the packaging graphics. Thus, a novel solution is required to solve this distortion problem whereby the barrier and sealing performance of the film is not diminished.
U.S. Pat. No. 5,500,282 discloses the use of a moisture barrier oriented polypropylene film containing a high crystalline polypropylene (HCPP) of isotactic content greater than 93% with an amount of polyterpene resin sufficient to improve moisture barrier properties. The inventors cite improvement in moisture barrier of less than or equal to 0.25 g/100 in 2/day/mil, but do not disclose any improvement in resistance to deformation due to oil absorption.
U.S. Pat. No. 5,667,902 discloses the use of a moisture barrier oriented polypropylene film containing a blend of high crystalline polypropylene of isotactic content greater than 93% with a second polypropylene of isotactic content between 90 and 93%, and a third component of a resin modifier of up to 9% to improve moisture barrier properties of the film. The inventors do not disclose any improvement in resistance to deformation due to oil absorption.
It is the objective of this invention to provide an economical solution to this package distortion problem caused by the oils of the food product.